Preparation And Properties Of Carbon Nanotubes Incorporated Lignin-based Nano-carbonfibers

Carbon fiber is first used in military and aerospace fields because of its characteristics of high specific strength and high specific modulus.With the introduction of the concept of automobile lightweight,the market of carbon fiber has a better development prospect.However,with the shortage of petroleum energy,the production cost of polyacrylonitrile-based carbon fiber(PAN)is too high,which limits the further development and application of carbon fiber.Lignin,as the second largest biopolymer in nature,exists widely in plants and its price is very low.Because of its high carbon content,lignin as raw material is an ideal choice to produce low-cost carbon fiber.At present,the mechanical properties of lignin-based carbon fiber are poor,so it is difficult to meet the actual production needs.Carbon nanotubes(CNTs),as the ideal reinforcements in composites,have been successfully used to improve the mechanical properties of precursor fibers and carbon fibers.However,due to the poor compatibility between carbon nanotubes and biopolymers such as lignin,the mechanical properties of lignin-based carbon fibers with carbon nanotubes as fillers are not as expected.The main contents and achievements of this paper are as follows:1.Lignin-based nanofibers were spun by electrospinning in n-dimethylformamide(DMF)with lignin as raw material and poly(ethylene oxide)(PEO)as spinning aid.After pre-oxidation and carbonization,lignin-based carbon nanofibers were successfully prepared.The effects of PEO content and its total mass fraction on fiber morphology and diameter,the effects of heating rate of pre-oxidation and pre-oxidation temperature on fiber morphology,diameter and oxygen content,and the effect of carbonation temperature on fiber residual carbon content were investigated.The results show that when the content of PEO in spinning solution is 4% and the total mass fraction is 35%,the morphology of lignin-based nanofibers is good,the surface is smooth and there are no beads,and the diameter of about 555 nm can meet the needs of subsequent carbonation.The best pre-oxidation process is to heat the temperature from room temperature to 70 °C at the heating rate of 5 °C/min,and then to 250 °C at the heatingrate of 0.05 °C/min for 2 h.At this time,the morphology of the fiber membrane is good,there is no melting and adhesion between the fibers,the diameter is about 325 nm,and the highest oxygen content is about 33%.The optimum carbonation process of this experiment is that the heating rate of 5 °C/min is heated to 800 °C and kept for 2 h,and the residual carbon rate of the fiber is 45%.2.Using lignin as raw material and PEO as spinning aid,the solution with a total mass fraction of 35% and a mass ratio of lignin to PEO of 96:4 was prepared by dissolving in DMF,then the lignin/O-CNTs composite nanofibers were prepared by electrospinning after adding 0.5%,1.0%,1.5% and 2.0%(the weight of lignin)O-CNTs respectively into the solution,under ultrasonic dispersion at room temperature for 3 h.Then the heating rate of 5 °C/min was heated from room temperature to 70 °C,then the heating rate of 0.05 °C/min was heated to 250 °C and preoxidized for 2 h.Finally,the lignin/O-CNTs composite carbon nanofibers was prepared by carbonization at the heating rate of 5 °C/min to 800 °C for 2 h.The morphology and structure of lignin/O-CNTs composite nanofibers and carbon nanofibers were characterized by scanning electron microscope(SEM),thermogravimetric analysis(TG),X-ray diffraction(XRD)and Raman spectrometer(Raman)and their mechanical properties were tested.The results show that with the increase of O-CNTs content,the morphology of the fiber changes from smooth to rough,and the diameter of the fiber gradually increases and the distribution is uneven.The addition of O-CNTs did not meet the expectation of strengthening the mechanical properties of carbon nanofibers.Only when the composite carbon fibers with Ts were added,the tensile strength of the composite carbon fibers was10% higher than that of the precursor.A small amount of O-CNTs(less than 1.0%)can improve the structural order of lignin-based carbon nanofibers.3.A novel amino-crosslinked polyphosphonitrile carbon nanotube(PZAA@CNTs)was prepared.Then,using lignin as raw material and PEO as spinning aid,the spinning solution with PEO content of 4% and total mass fraction of 35% was prepared by dissolving them in DMF.Then the lignin/PZAA@CNTs composite nanofibers were prepared by electrospinning after adding 0.5%,1.0%,1.5% and 2.0%(the weight of lignin)PZAA@CNTs respectively into the solution,under ultrasonic dispersion at roomtemperature for 3 h.Then the heating rate of 5 °C/min was heated from room temperature to 70 °C,then the heating rate of 0.05 °C/min was heated to 250 °C and preoxidized for 2h.Finally,the lignin/PZAA@CNTs composite carbon nanofibers was prepared by carbonization at the heating rate of 5 °C/min to 800 °C for 2 h.The morphology and structure of lignin/PZAA@CNTs composite nanofibers and carbon nanofibers were characterized by SEM,TG,XRD and Raman,and their mechanical properties were tested.The results show that compared with O-CNTs,the interfacial interaction between carbon nanotubes and lignin molecules is enhanced by the addition of PZAA@CNTs,and the mechanical properties of lignin-based nanofibers are enhanced at all stages.The tensile strength and elastic modulus of the composite carbon fiber containing1.0%PZAA@CNTs were 54% and 44% higher than those of the precursor,respectively.